1. Field of the Invention
This invention relates to a view finder optical system for a single lens or twin lens reflex camera.
2. Description of the Prior Art
One type of view finder optical system of the type which has been widely used in single lens or twin lens reflex cameras has a pentagonal dach prism above a focusing plate and an eyepiece behind the pentagonal dach prism. The pentagonal dach prism is capable of changing the inverted image to be photographed appearing on the focusing plate to a corresponding erect image which can be seen through the eyepiece. As is well known, the magnification of the view finder is determined by the ratio of the focal length of the taking lens to the focal length of the eyepiece, and therefore the magnification of the view finder increases inversely with the focal length of the eyepiece.
However, as a natural consequence of using a cemented type eyepiece of short focal length, there is entailed the necessity of shortening the distance between the eyepiece and the focal plate. On the other hand, the size of the pentagonal dach prism cannot be reduced too much because undesired reduction of the viewing area would result. Therefore, the distance between the eyepiece and focusing plate cannot be reduced to less than a certain allowable length which is defined by the size of the intervening pentagonal dach prism. This means that in a view finder optical system including a pentagonal dach prism, it is very difficult to increase the magnification of the view finder by simply using a cemented type eyepiece of short focal length. One way to solve this difficulty is to use a special eyepiece having a short focal length as well as a desired back focal length in the view finder optical system. Such a special eyepiece, however, consists of a large number of lens elements, and is accordingly expensive.